Abstract
Scanning tunneling microscopy combined with terahertz (THz) electromagnetic pulses and its related technologies have developed remarkably. This technology has atomic-level spatial resolution in an ultrahigh vacuum and low-temperature environment, and it measures the electrical dynamical behavior of a sample’s surface with femtosecond temporal resolution. In particular, it has been used to image the diffusion and relaxation dynamics of electrons in real time and real space and even instantaneously control molecular motions. In this Perspective, we focus on recent progress in research and development of ultrafast time-resolved THz scanning tunneling microscopy and its application to materials research.
Highlights
Since its invention in 1982,1 scanning tunneling microscopy (STM) has been used to image the electronic states of material surfaces with atomic resolution.[2,3] In STM, electrons, driven by an applied bias voltage, pass through a tunnel junction that forms when the metallic tip is set in very close proximity to a conductive sample [Figs. 1(a) and (b)]
When the tip is sharpened to an atomic-scale apex like in Fig. 1(a), STM is able to produce two-dimensional images of individual atoms of solid surfaces and map the local density of state (LDOS) on metallic surfaces.[4,5]
THz-STM was first reported by Cocker et al in 2013, where it was used for imaging of highly ordered pyrolytic graphite (HOPG) and semiconductor nanodots under ambient conditions.[20]
Summary
Since its invention in 1982,1 scanning tunneling microscopy (STM) has been used to image the electronic states of material surfaces with atomic resolution.[2,3] In STM, electrons, driven by an applied bias voltage, pass through a tunnel junction that forms when the metallic tip is set in very close proximity to a conductive sample [Figs. 1(a) and (b)]. As a way around this limitation, ultrashort optical pulses can be irradiated on the tunnel junction as an alternative to application of an electronic bias This method has recently been used to modulate the I–V curves transiently and measure carrier and spin dynamics with femtosecond time resolution and nanoscale spatial resolution.[19–26]. The term “terahertz (THz) light” refers to electromagnetic waves with frequencies ranging from 0.1 to 10 THz, energies from 0.4 to 41 meV, wavelengths from 30 μm to 3 mm, and wavenumbers from 3.3 to 333 cm−1.27–29 Many elementary excitations that are distinct physical properties of solids exist in this frequency: for example, superconducting gap,[30,31] phonons,[32–34] spin resonances,[35–37] plasma frequencies,[38,39] electron binding energies of impurities,[40] and binding energies of excitons in semiconductors.[41,42] This makes the THz frequency region very attractive from the viewpoint of materials science. In this Perspective, we review recent progress in research and development of STM based on THz pulses
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